In computer-based image composition and editing systems, which store imagery data in digital format, it is common practice to selectively combine plural images by merging or inserting a first image, termed the `foreground` image, into or on top of a second image termed the `background` image. A typical example is the extraction of an image of a human subject that has been taken against a prescribed color backdrop (usually blue) and inserting the extracted image into a scenic background (for example, an image of the U.S. Capitol building in Washington D.C. ). Because the images are different, along the edge or border between the background image and the foreground image, an undesirable boundary artifact is created. This anomaly is particularly pronounced where the border has an arbitrary shape (as in the case of a human subject), due to the quantized format of the image data base and the finite matrix configuration of the pixels of the display used to combine the images. One possible mechanism for reducing the artifact is to locally blur the edge of the `pasted` foreground, in order to `soften` the effect of the image discontinuity. This `softening` or `electronic air-brushing` is carried out by an operator using a zoom feature of image editing software, so that it is both labor-intensive and inexact (a pasted foreground image with a blurred border).